brids might finally achieve a thoro exchange of genes thruout the whole of the population. Then a sample taken from any part of the range would vary within the same wide but uniform limits typical of every other sample of the population. It would then satisfy our concept of a species, for it would have a common heredity.
It is this problem, the attainment of homogeneity out of a hybrid population, that has become the immigration problem of the American people in the last half-century. Whether homogeneity is a biologic virtue or not, the statesmen have shown themselves good taxonomists in their insistence that we cannot become a true species until barriers are erected to protect us from continued contributions of parental stock. Whether in peoples or insects, the melting pot cannot blend diverse materials that pour in too rapidly.
Now it becomes obvious why hybrid populations cannot often give rise to new species. Such inter-specific hybrids must usually arise in limited areas between the parental species. As long as the area is limited and the parents are close at hand, the hybrids will continue to be hybrids of every shade and extreme and intermediate combination of parental characters. On theoretic grounds, it would appear that hybrids may become species only when geographically removed or in some other way isolated from the parental stock. These circumstances would seem so rare that we cannot believe that hybrids account for the origin of many of the species with which we are acquainted in the field today.
And yet, there is one group of species among the Cynipidae which would seem to have had hybrid origin. These species occupy that very portion of the northeastern United States in which so many of our biologic studies have been pursued. Cynips erinacei, to which repeated reference has already been made, is the most certain of these cases in the genus Cynips.
Erinacei, it will be recalled, is a highly variable species occupying about 500,000 square miles of the area which we have just defined. It is unique among Cynips in the extent of its individual variation. The extreme individuals of the group have previously been considered representatives of two distinct species, but the specific unity of erinacei is affirmed by the existence of every type of intergrade between these extreme individuals, and by the occurrence of all these varia-
